Harry Potter and the Prisoner of Azkaban 

Harry Potter has to sneak back to Hogwarts, after accidentally inflating his horrible Aunt Petunia. But once there everyone is whispering about a prizoner who has escaped from the famous wizard prizon, Azkaban. His name is Sirius Black, and as a follower of Lord Voldemort he is determined to track Harry Potter down - even if it means laying siege to the very walls of Hogwarts!